302 METALLURGY 03? IRON AOTD STEEL.
. SEO. XVk.—Effect .of -work, on the structure of soft steel forging steel.—Steel as nsnally cast, cooling slowly from the liquid state with no work done upon it, forms in crystals and shows, in, general the same structure throughout. The outer skin has a structure different from the rest of the mass/ as it cools quickly and is under heavy strains as long as any of the metal is hot, and there is also an area of abnormal crystallization at the top of the ingot-due to segregation, but the greater part of an ingot is of the same general crystalline character. Rolling tends to break up this "grain and prevent further growth during the process, but immediately after cessation of work the formation of grains begins and continues until the metal has cooled to the lower critical point. Hence it is evident that the lower the temperature to- which steel -is worked the more broken up the structure will be, but on the other hand if the rolling be continued below the critical point, the effect of cold work will be shown and strains will be set up which will make the piece unfit for use without annealing. Consequently it is necessary to stop the work somewhat above the critical point and in practice with large pieces it is customary to finish some 150° G. to 200° C. above this point, since the metal becomes so stiff at the lower temperature that the wear and tear on. the rolls is excessive. In blooms, billets and such hard steels as are to be reheated for hardening, the need of an extremely low finishing temperature is not so evident. If the grain be reasonably fine, the metal is solid and dense, -and the crystallization of the steel when put in service will be determined by the final heat treatment. This will be taken up'more in detail in Section XYm. It would appear that the smaller the piece the finer the grain, and this arises partly from the' necessity of finishing a 'large piece while the center is still hot and partly from the slower rate of cooling of the large piece. In No." 3Iff Fig. XV-G-, is shown the micro-structure of a low-carbon ingot magnified 20 diameters and-in ISTos. 38 and 39 the same grade of steel rolled into 1" rounds and magnified 75 diameters. These last two are the center and .outside,respectively of the same piece and show the effect of a high temperature in burning the carbon of the steel near the surface. The dark.element in No. 3'8 is pearlite, the light is ferrite. It will be noticed that very little pearlite is shown in No. 39. This is in accordance with the explanation in Section XVm, where it is shown that if the carbon wene partly burned away it would leave just so much less cementife